Various methods are already known for determining a reliable vehicle reference speed in motor vehicle brake control systems, drive slip controllers and driving dynamics controllers for motor vehicles.
According to DE 43 14 830 A1, which is incorporated by reference, this is based solely on an analysis of wheel speeds. If further sensors are available in the vehicle, the same can also be used for reference speed determination. WO 02/103366, which is incorporated by reference, thus discloses the determination of the vehicle reference speed in an ESP/ESC controller by means of a longitudinal acceleration sensor, the wheel revolution rate sensors and the position determination by means of a GPS sensor. DE 10 2008 045 619 A1, which is incorporated by reference, also processes the position information determined by means of a camera. A determination method emerges from DE 10 2004 060 677 A1, which is incorporated by reference, in which the speed over the ground is determined by means of an optical method. WO 2011 023 591 A1 and DE 10 259 272 A1, which are incorporated by reference, are concerned with the combination of wheel speed information, a model for the longitudinal acceleration and a wheel dynamics model. The wheel dynamics model and the longitudinal acceleration model estimate or determine the acceleration of the vehicle based on the wheel displacement.
With a method purely based on slip-free rolling wheels of the non-driven axle, situations can arise in which the vehicle speed cannot be correctly determined. A vehicle reference speed signal is then not representative of the actual vehicle speed during some braking processes or in a case of drive slip. This problem exists more particularly in all-wheel drive vehicles in case of drive slip or drag torque. This can often result in the estimated speed being underestimated in the braking situation for all types of vehicle and overestimated in the drive situation for all-wheel drive vehicles.
By including the acceleration signal and engine torque it is possible to make the estimated speed plausible. However, the engine torque signal is not always available in a braked situation, because the engine can be decoupled. In addition, the drag torque signal does not have a comparable high quality to the engine torque signal in the drive situation. Moreover, the engine torque cannot be fully transferred to the road, because the wheels can spin or lock. On the other hand, the acceleration signal of the inertial sensor system is subject to offset because of the long-term drift, the assembly inaccuracies and the road gradient. In addition the signal is affected by noise from axle oscillation and deceleration caused by inertia. There is no equivalent redundancy for the acceleration sensor.
The aforementioned DE 10 259 272 A1 is concerned with the problem of the fusion of signals having different reliability.
According to the described method, the vehicle speed is determined by including a plurality of available sensor signals whose influence on the computing result depends on weighting factors.